Solid-state cell and battery



able.

United States Patent SOLID-STATE CELL AND BATTERY Harry C. Lieb, Rockville Centre, and John A. DeTRosa,

Jamaica, N.Y., assignors to Leesona Corporation, a

corporation of Massachusetts No Drawing. Application April 22, 1958 Serial No. 730,059

This application is a continuation-in-part of our application Serial No. 526,055, filed August 2, 1955, and now abandoned.

This invention relates to-primary electro-chemical cells which consist entirely of solid components and to solidlife as compared to solid-state batteries heretofore avail- We have also found that our batteries are not affected adversely by wide variations in ambient tem- Accordingly, the primary object'of the pres-1.; .ent invention is to .provide an improved primaryelect-ro-- p eratures chemical cell which has a high voltage, high current drain, and a long shelf life, which may be employed in the construction of solid-state batteries having the same desirable characten'stics. A further object of the invention is ,to provide an electro-chemical cell of such character that it can be constructed, and will perform efficiently, in difierent sizes, so that it may be equally capable of serving as the basis for a miniature battery or one of large size. It is also an object of the present invention to provide a primary electro-chemical cell in which the usable current drain does not polarize the cell, and thus lead to a gradual reduction of its voltage and useful life.

The functional characteristics of a solid-state battery depend to a great extent on the composition of the solid electrolyte which serves as an ionic conductor between the, electrodes of the individual cells from which the hatteryis assembled. The ionic conductivity of the elecr trolyte, or the mobility of ions therethrough, determines the rate at which the chemical changes Within the cell occur, and also the nature of the changes, and thus affect the useful life, the voltage and current characteristics of the cell. In most instances, it is desirable that the composition of the solid electrolyte be such that the ionic conductivity is at a high level but, at the same time, such that the cell will not be shorted through the electrolyte. It is also necessary that the composition of the electrolyte be such that polarization of the cell is minimized, and preferably entirely avoided, and that chem ical action within the celldoesnothave side effects which prematurely destroy or incapacitate any component of the cell. We have found that a solid electrolyte may b sub t nt a improved .6 1 th f re oing respects, and to an unexpected degree, byincorporating tellurium into theelectrolyte, in certain percentages by weight of electrolyte. More specifically, we have found that the addition of 3 to tellurium to a solid electrolyte results in a substantial and permanent increase in the open circuit voltage and in the useful current producediby a solid-state electro-chemical cell. 'It has beenfound thatthe improvement in the ionic conductivity of a solid electrolyte by tellurium is confined largely to theaddition: ofabout 310,%, and that amounts outside of this range have inappreciable effect, so that the advantages achieved by the addition of tellurium apparently cannot be attributed to the conductivity of tellurium itself. .It

has also been found that the optimum improvement achieved by the addition of tellurium occurs when it is used in the amount of 5%, by weight, of the electrolyte.

The precise reasons why the addition of tellurium I0 the electrolyte in a solid-state cell has the described effects are not clearly understood. Presently accepted theory regarding solid-state cells postulates that a solid electrolyte composition (an ionic conductor) has a defeet-lattice structure with the normal ions moving relatively, freely as interstitial atoms, and has vacant lattice points-or holes normally occupied by the moving ions The addition of tellurium to such an electrolyte apparently results in the formation of compounds which'fun .ther alter the lattice'structure to enhance the characteristics which initially made it suitable as a solid electrolyte, 1 or make-more readily available ions which migrate into However, the present the vacant lattice points or holes. invention is not limited to any particular theory or explanation of the efliect of tellurium in the electrolyte, nor to the specific illustrations of the invention which are set forth below in order to further illustrate and explain it.

The advantageous enhancing of the ionic conductivity ()f a solid electrolyte by the addition of tellurium is shown below bythe comparison of the electrical conductivity of solid silver chloride, a common solid-state electrolyte, in its pure form, with the conductivity of the same salt with tellurium added.

The comparison-was carried out as follows: Y

Molten, pure, silver chloride was poured between two silver strips held 3 mm. apart in a porcelain combustion boat. The boat was then heated sufiiciently to smooth out the molten silver chloride and allowed to adhere to the silver strips. Kel-F coated wire was mechanically fixed to the silver strips. The resistance of the solid solutions was measured on a voltohmyst meter at three temperatures. The results are indicated below:

Solid Solution Temperature, Resistance,

' 0. ohms.

i I A 24 1x10 PurengoLn, 7. 7x10 l 200 4.6 '1(l 24 X 0 AgOl+0.5% Te 100 1. 8X10 AgOl+5% Te 1 100 2. 3x10 It was noticed that the cell resistances did not in,-

crease from the'improved values which. theyattained; as

a result of the heating when the cells were subsequently cooled. Such heating, therefore, represents a manner in, which; the solid electrolyte cells may be further inn proved before being put to use.

Itis' apparent from the foregoing'results. that'theaddiition of tellurium to the silver chloride electrolyte: sub.- stantially increased the ionic conductivity, and it was further noted that the increase in ionicconductivity was not of such nature that shorting through the. electrolyte occurredwhen the electrolyte'jvas employed in a thin layer (1 mil) between anode and cathode of a cell.

The nature and best mode of application of the invention may be further understood from the following examples:

Example I A primary, solid-state electro-chemical cell was produced by first fusing together into a solid solution 19 grams of silver chloride and 1 gram of tellurium.

A number of cells having a solid electrolyte comprising silver bromide and varying percentages of tellurium, by weight, were made as follows. The electrolyte com- Sheets of 5 mil. silver were fuse-coated on one side with 5 positions for the cells were prepared by fusing silver the solution of silver chloride and tellurium. Discs of bromide with the following proportions of tellurium by A" in diameter were then punched from the coated weight, of the total respectively, .01%, 0.1%, 1.0%, 3.0%,

sheet. The electrolyte side of the discs was thereupon 5.0%, 7.0% and 10.0%. In addition, one cell was precoated with a solution of 2.5 gram of CuCl in mil. of pared with an electrolyte of pure silver bromide. These methanol in which 1 gram of 10% Glydag B (colloidal 10 various electrolyte compositions were applied by rubbing graphite in 1,3 butylene glycol) was dispersed. The them onto a piece of 5 mil. silver sheet, which had been discs were then dried at 110 C. The primary cell thus previously cleaned with a 35% nitric acid solution and a prepared comprised a silver anode, a solid-state electrowash of de-ionized water, while the sheets were held over 'lyte of silver chloride and tellun'urn and a cathode of 1 flame. The electrolyte coating was made to a thickness CuCh-i-C. Six cells of this kind in series were stacked 5 of 2-3 mils. The cathodes of the series were prepared in a Lucite tube, and uncoated silver discs were placed at by impregnating filter paper with a 5% Aquadag diseach end of the stack to serve as electrodes. Silver persion having the following composition: 11 parts wires, soldered to these discs, protruded through the Aquadag (22% graphite), 33 parts of de-ionized water,

closed ends of the Lucite tube for connection to leads. and 2 drops Tween 80. After drying, this impregnated The individual cells of the battery thus formed were paper was further impregnated with a solution of pressed firmly in contact with one another. cupric bromide in methanol. The impregnated paper Four other sets of six cells each were similarly formed was then dried and cut into discs. These discs were then into batteries except that the composition of the elecfixed to the center of the electrolyte-coated side of the trolyte of each set was varied, in one case pure silver 25 silver disc after a 1" copper disc had been placed on the chloride being used and in the others a combination of other side of the paper. The discs were held firmly in silver chloride with .01% tellurium, .1% tellurium, and contact with one another. The cells thus formed ex- .5% tellurium, respectively. The five batteries thus conhibited the electrical properties indicated below:

Meter Shunts, Ohms. Current at Cell Re- Load Equal Electrolyte O.C.V. slstauce to Cell 10 10 10 10 Resistance,

mtcroamp.

Pure AgBr 0.80+ 0. 80+ 0.s0+ 0.80+ 0. 73 114K 3 AgBr+0.0l% Te"--. 0. 80+ 0.80+ 0.80- .75 .40 599K 1 AgBr+0.1% 'Ie .80 .80 .79 .70 .57 323K 1-2 AgBr+l% Te 80 79 77 74 00 178K 2 structed were tested with the following results:

In a further application of the invention, anelectrolyte comprising silver bromide plus 5% by weight, of tellurium was flame-coated on, one side of a 5 mil. silver sheet in a layer of about 1 mil. thickness. A disc of about diameter was punched out from this coated sheet. A good grade of semi-quantitative (No. 2) filter paper was then saturated with a solution consisting of 2.5% gram cupric bromide and one gram Dag" dispersion No. 154 (20% iso-propanol dispersion of colloidal graphite) in 10 mil. of methanol. The paper was then dried and a diameter disc was punched therefrom. The silver disc coated with the electrolyte and the cupric bromide paper disc were placed firmly in contact with one another. The following voltage readings were obtained:

. Shunts, Ohms 0.0.v.

The cell resistance was measured by applying an external load until the O.C.V. was reduced to one-half, making the load resistance equal to the cell resistance. Cells were also observed for the effect of prolonged exposure to air and for shelf life characteristics. No substantial change in the characteristics of the cell was noted following prolonged exposure in the air, and, characteristically, the cell comprising the electrolyte with 5%, by weight, of tellurium after 31 days exhibited an O.C.V. of .74, cell resistance (Ri) of 1.8 K and current at Ri of 80 microamperes. A study of the temperature dependency of these cells was made at 50 C., 29 C., and at about C., the results indicating that the cells had outstanding performance characteristics throughout this temperature range. The results with the cell comprising the electrolyte of silver bromide with 5%, by weight, of tellurium, and compared with that having the electrolyte of silver bromide alone, being indicative of this.

1 Too low for detection on Simpson meter. Example I V A primary, solid-state cell was made by preparing a solid electrolyte consisting of AgI with, respectively, 5%, 10%, 12.5% and 15% tellurium, by weight. Sheets of 5 m silver were fuse-coated on one side with these ner described immediately above.

solid electrolyte compositions. Thereafter, discs of about one inch in diameter were punched from the coated sheet. The side having the solid electrolyte was then coated with a solution consisting of CuBr and a dispersion of colloidal graphite. This provided the counter electrode with the conductive graphite required to tap this electrode. The cells thus prepared comprised a silver anode, a solid-state electrolyte of AgI and tellurium, and a cathode of CuBr Upon test it was found that the cell having the electrolyte of AgI plus 5% tellurium at room temperature showed a voltage of .72 through a resistance of ohms at room temperature, thecell having the 10% tellurium in the electrolyte showed a voltage of .75 through aresistance of 10 ohms, and that having 12.5% tellurium was .72 volt through a resistance of 10 ohms. It was found that electronic conduction occurred through the electrolyte containing the or more tellurium.

Due to the instability of the AgI electrolyte, the cell CuBr and colloidal graphite.

It will be appreciated that other techniques may be employed in constructing the primary cells of the kind herein disclosed. For example, a primary cell was constructed by rolling a layer of silver bromide and 5% tellurium onto a sheet of silver. This silver sheet was approximately .002 inch in thickness and about 2" wide. The layer of silver bromide plus 5% tellurium was applied in a thickness of .002 inch. The silver bromide plus tellurium electrolyte material was clad onto the silver according to techniques which are understood in the art. The silver side of this two-layer sheet was then sprayed with a solution of an adhesive binder (epoxy resin in this case) and colloidal graphite to provide an electronically conductive film .001 inch in thickness which serves to tap the silver (anode) and protect it from attack by the halide constituent of the cathode of an adjacent cell when a multiplicity of cells are assembled into a battery.

The solid electrolyte material which was clad to the silver was prepared by melting silver bromide powder in a crucible and thereafter mixing in the 5% by weight of tellurium. The two components are thus fused together and then cast into ingots. The ingots thus provided are handled in the usual way in applying the silver bromide plus tellurium to the silver sheet.

The counter electrode (cathode) was prepared by making a concentrated solution of copper bromide (CuBr in hot water.. This was done by adding 67 grams of copper bromide in sufiicient hot water to dissolve it, which was approximately 100 cubic centimeters. To this was added 150 grams of an aqueous dispersion of colloidal graphite. The solids content of this dispersion was about 22%, so that the dispersion had the consistency of a paste, the density being about 9 pounds per gallon. The particle size of the colloidal graphite was about 1 micron or less.

Upon the addition of the colloidal dispersion, the mixture had a pasty consistency. This mixture of graphite and copper bromide was used to print the counter electrode upon the solid electrolyte of silver bromide plus tellurium. The counter electrode was thereupon dried and cells punched from the strip. The characteristics of the cells formed in this manner were at least as good as the silver bromide plus tellurium cells described above. It was found that marked improvement resulted from forming the counter electrode in the man- For example, the use of the aqueous dispersion of colloidal graphite having size or were dispersed in theorganic solvents.

removed or displaced by chemicals. It was also found that the dispersions of colloidal graphite in liquids which included organic solvents did not dry out to form a a film having the same skin appearance as the better performing electrode film deposited from the aqueous dispersion having the particles of one micron or less. A further advantage following from the use of the aqueous dispersion described is'that the electrode exhibited improved electrical characteristics and made possible a higher current drain from the cell.

Cells of the kind described in the examples above were also formed with silver electrodes to which the solid electrolyte was laminated under pressure. For example a biclad material was prepared by fusing the silver bromide and tellurium onto a silver sheet using 102 mgm. of this solid electrolyte for each square centimeter of silver. These sections were then placed between hot plates whose faces were formed of stainless steel, and one of these plates had a countersunk depression of 10 mils. 250300 C., after placing the biclad material between them and pressures varying from to 1000 psi, in different instances, were applied. This resulted in a smooth dense laminated material comprising a sheet of silver and an overlay of silver bromide and tellurium, which served to provide the silver anode and solid electrolyte of a cell.

A preferred form of the invention, therefore, comprises the use of a counter electrode which is deposited from a solution comprising an aqueous dispersion of colloidal graphite having a particle size of one micron or less.

The cell system employing silver bromide and tellurium as the electrolyte is the preferred one of those described above, in that the silver bromide is more conductive than the chloride, it has a higher E.M.F., and the cupric bromide is less hygroscopic than the chloride.

It will be appreciated that any of the silver halide salts may be used with tellurium as an electrolyte and will have improved properties over the halide salt used alone. For most applications, the bromides and chlorides are more suitable, but silver iodide may be used in some instances, since it is generally considered a better ionic conductor than the bromide. However, in most cases it is less usable than the bromide and chloride because of its instability. Other solid-state electrolytes which may be advantageously modified by the addition of tellurium include, for example, the halides of mercury, antimony, bismuth, and lead. composition of the electrodes may be varied from those described. As understood in the art, metals such as strontium, barium, rubidium, and others may be employed as anodes and that gaseous elements absorbed in surface layers of a solid'material, and oxidizing salts may be employed as cathodes.

Having thus described the invention what is claimed as new is:

1. An electro-chemical cell having a solid electrolyte consisting essentially of a halide selected from the group consisting of chlorides, bromides and iodides and 340%, by weight, of tellurium.

2. An electro-chemical cell having a silver anode, and a solid electrolyte consisting essentially of a halide selected from the group consisting of chlorides, bromides The two plates were heated to about It will also be appreciated that the V 4. An electro-chcmical cell having a silver anode, a cupric bromide cathode and a solid electrolyte consisting essentially of silver bromide and 3-10%, by weight, of tellurium.

5. A primary cell having a solid electrolyte and spaced apart electrodes in contact therewith, said electrolyte consisting essentially of a halide selected from the group consisting of chlorides, bromides and iodides, and 13-10%, by weight, of tellurium.

6. A battery comprising an electrically interconnected assembly of cells of the kind claimed in claim 1.

7. A method of preparing an electrolyte comprising melting together a solid crystalline ionic conductor selected from the group consisting of chlorides, bromides and iodides, and 310%, by weight, of tellurium.

8 8. An electro-chemical cell having a solid electrolyte consisting essentially of an iodide and more than 3% to less than 15% tellurium.

References Cited in the file of this patent UNITED STATES PATENTS 2,418,792 Riggs Apr. 8, 1947 2,605,300 Shirland July 29, 1952 2,658,099 Basset Nov. 3, 1953 2,690,465 Broder Sept. 28, 1954 2,700,064 Akerman Ian. 18, 1955 2,715,150 Gritman et a1 Aug. 9, 1955 2,715,151 Gritman et al Aug. 9, 1955 2,793,244 Van Der Grinten May 21, 1957 

1. AN ELECTRO-CHEMICAL CELL HAVING A SOLID ELECTROLYTE CONSISTING ESSENTIALLY OF A HALIDE SELECTED FROM THE GROUP CONSISTING OF CHLORIDES, BROMIDES AND IODIDES AND 3-10%, BY WEIGHT, OF TELLURIUM. 